drm/i915/ehl: Allow combo PHY A to drive a third external display

EHL has a mux on combo PHY A that allows it to be driven either by an
internal display (DDI-A or DSI DPHY) or by an external display (DDI-D).
This is a motherboard design decision that can not be changed on the
fly.  Unfortunately there are no strap registers that allow us to detect
the board configuration directly, so let's use the VBT to try to figure
it out and program the mux accordingly.

For now if we run across a broken VBT that tries to claim that PHY A
is attached to both internal and external displays at the same time,
we'll resolve the conflict in favor of the internal display.  To help
debug these kind of bad VBT's, let's also add a quick DRM_DEBUG message
during child device parsing so that it's easier to understand these
cases if they show up in bug reports.

v2:
 - Confirmed that VBT's dvo port refers to the DDI and not the PHY.
   Thus we can check more explicitly for (ddi_d && !(ddi_a || dsi)).  If
   a bad VBT contradicts itself, let internal display win.  (Ville)

v3:
 - Switch condition from !IS_ICELAKE to IS_ELKHARTLAKE.  Although the
   convention is usually to assume that future platforms will inherit
   all current platform behavior, this feels more like a one-platform
   quirk.  (Ville)
 - Update commit message to describe what we do if/when we encounter
   broken VBT's, and note that the new debug print during child device
   parsing is intentional.

Cc: Clint Taylor <Clinton.A.Taylor@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618175131.9139-1-matthew.d.roper@intel.com